The second of the ten plagues G‑d brought upon the Egyptians was the plague of frogs. The Parshah tells that upon striking the Nile, the frogs emerged from the Nile and "covered the Land of Egypt."
Rashi, the principle Bible commentator, notes that the Torah employs the singular word for frog-tzefardeah, rather then the plural tzefard'im. Rashi's simple explanation for this anomaly is that the word really does not mean frog, but the swarming of the frog species.
Rashi's preferred explanation, however, is the one he cites from the Midrash. In fact, the Midrash states, there was only one frog that emerged from the Nile . When the Egyptians saw this one frog emerge they began to strike it, whereupon "streams" of frogs came out of the one "super frog" that then swarmed throughout the Land of Egypt.